Swig c++ w/ Java loses type on polymorphic callback functions

Question

Question: Why is my C++ swigged object losing its type when passed to a Java callback function?

Setup: I've taken the Swig Java example for doing callbacks and added an object to be passed to the callback run(Parent p). The callback works as expected but when I pass a Child object the Java seems to lose its type and think its of type Parent when it should be Child. This is based on the Swig java callback example.

System Info: Ubuntu 8.04 w/ Swig 1.3.33 - on the off chance the latest Swig made a difference I also tested 1.3.39 - which had no effect.

Outputs:

bash$ java -Djava.library.path=. runme
Adding and calling a normal C++ callback
----------------------------------------
Callback::run(5Child)
Callback::~Callback()

Adding and calling a Java callback
------------------------------------
JavaCallback.run(Parent)
Callback::run(5Child)
Callback::~Callback()

As you can see in the outputs - the object is really of type Child - but its Java class name is Parent - which is wrong...

If you look in the Java callback run(Parent p) you can see where I'm fetching the Java class, and Java really does think this object is of type Parent - trying to cast this to Child will throw ClassCastException as expected.

Code:

/* File : example.i */
%module(directors="1") example
%{
#include "example.h"
%}

%include "std_string.i"

/* turn on director wrapping Callback */
%feature("director") Callback;

%include "example.h"




/* File : example.h */
#include <string>
#include <cstdio>
#include <iostream>
#include <typeinfo>

class Parent {
public:
    virtual const char* getName() {
        return typeid(*this).name();
    }
};


class Child : virtual public Parent {
};



class Callback {
public:
    virtual ~Callback() { std::cout << "Callback::~Callback()" << std:: endl; }
    virtual void run(Parent& p) { std::cout << "Callback::run(" << p.getName() << ")" << std::endl; }
};


class Caller {
private:
    Callback *_callback;
public:
    Caller(): _callback(0) {}
    ~Caller() { delCallback(); }
    void delCallback() { delete _callback; _callback = 0; }
    void setCallback(Callback *cb) { delCallback(); _callback = cb; }
    void call() {
        Parent *p = new Child();
        if (_callback) 
            _callback->run(*p);
        delete p;
    }
};



/* File: runme.java */
public class runme
{
  static {
    try {
        System.loadLibrary("example");
    } catch (UnsatisfiedLinkError e) {
      System.err.println("Native code library failed to load. See the chapter on Dynamic Linking Problems in the SWIG Java documentation for help.\n" + e);
      System.exit(1);
    }
  }

  public static void main(String[] args)
  {
    System.out.println("Adding and calling a normal C++ callback");
    System.out.println("----------------------------------------");

    Caller              caller = new Caller();
    Callback            callback = new Callback();

    caller.setCallback(callback);
    caller.call();
    caller.delCallback();

    callback = new JavaCallback();

    System.out.println();
    System.out.println("Adding and calling a Java callback");
    System.out.println("------------------------------------");

    caller.setCallback(callback);
    caller.call();
    caller.delCallback();

    // Test that a double delete does not occur as the object has already been deleted from the C++ layer.
    // Note that the garbage collector can also call the delete() method via the finalizer (callback.finalize())
    // at any point after here.
    callback.delete();

    System.out.println();
    System.out.println("java exit");
  }
}

class JavaCallback extends Callback
{
  public JavaCallback()
  {
    super();
  }

  public void run(Parent p)
  {
    System.out.println("JavaCallback.run("+p.getClass().getSimpleName()+")");
    super.run(p);
  }
}




# File: Makefile
TOP        = ../..
SWIG       = $(TOP)/../preinst-swig
CXXSRCS    = example.cxx
TARGET     = example
INTERFACE  = example.i
SWIGOPT    =

all::   java

java::
    $(MAKE) -f $(TOP)/Makefile CXXSRCS='$(CXXSRCS)' SWIG='$(SWIG)' \
    SWIGOPT='$(SWIGOPT)' TARGET='$(TARGET)' INTERFACE='$(INTERFACE)' java_cpp
    javac *.java

clean::
    $(MAKE) -f $(TOP)/Makefile java_clean

check: all

This might be a bug in Swig - but I'm hoping that this is my being stupid with C++ types/casting...

Any thoughts would be greatly appreciated!

Answer 1

After digging around on this problem for the weekend I guess this is a "common" problem that Swig has between C++ classes and Java. The issue is called downcasting and is a common problem of directors. Unfortunately the directors don't seem to be able to handle even this simple case. I've tried every combination of director - like below

%feature("director") Callback;
%feature("director") Parent;
%feature("director") Child;

None of that seemed to help at all but doing the following hack worked ok:

class Callback {
public:
    virtual ~Callback() { std::cout << "Callback::~Callback()" << std:: endl; }
    virtual void run(Parent& p) {
        std::cout << "Callback::run1(" << p.getName() << ")\n";
    }

    virtual void run(Child& c) {
        std::cout << "Callback::run2(" << c.getName() << ")\n";
    }
};

Then in the java class for whatever subtype you need the overload irons itself out.

class JavaCallback extends Callback
{
  public void run(Child p)
  {
    out.p("JavaCallback.run("+p.getClass().getSimpleName()+")");
    out.p("p.getName() = "+p.getName());
    super.run(p);
  }
}

Then magically the output works

bash$ java -Djava.library.path=. runme
Adding and calling a normal C++ callback
----------------------------------------
make child
child type class Parent
Callback::run2(5Child)
Callback::~Callback()
Adding and calling a Java callback
------------------------------------
JavaCallback.run(Child)
p.getName() = 5Child
Callback::run2(5Child)
Callback::~Callback()
java exit

There should probably be a better way to do this, but none of the Swig documentation presented me a clear example of how to do this properly. There was some really impressive code in the libsbml library which might help people create downcasting typemaps which fix the problem, but that was proving very complex for little output... Anyway this was simple and easy.

If anyone can figure out an easy (human) solution I'd be interested in hearing about it.